Significance The development of an effective contraceptive vaccine could be useful in countries where medical care is limited. Objectives In this study the sperm antigen MacY19TT, with synthetic muramyl dipeptide as adjuvant was administered by intramuscular injection, followed by weekly blood collections for measurement of antibody titers. Two treatment groups and one control group of 15 monkeys each, received either antigen with adjuvant, antigen without adjuvant or antigen without active site in vehicle for treatment groups 1 and 2 and control group 3 respectively. A primary injection for all dose groups followed by two booster treatments spaced approximately one month apart was to be administered. Results Approximately one month after primary treatment, several animals in groups 1 and 2 had a thickened mass at the injection site. The first booster was administered according to schedule to all animals, however additional animals from that first reported developed either thickened masses or abcesses at the injection sites. The thickened masses first reported from groups 1 and 2 turned into abcesses about one to two weeks after the first booster injection was administered. The second booster injection was delayed due to the severity and number of reported abcesses in the treatment groups. In addition, the serum antibody titers were too high to administer the second booster treatment. Additional serum antibody tests conducted by Dr. Goldberg revealed antibodies not binding to, immobilizing or agglutinating sperm. These results differed from the pilot study and the mating phase of the study was terminated. Future Directions Due to the unexpected presence and severity of abcesses in the monkeys and differing results in the antibody analyses, Dr. Goldberg will reformulate the antigen and assess its sperm binding capabilities prior to initiating another fertility trial. KEYWORDS immuno-contraception, sperm antigens, vaccine